Organic polymers are used for charge transport in devices such as organic light-emitting diodes (OLED) and organic electrophotographic photoreceptors or photoconductors (OPC). In OLEDs charge is injected from an electrode into a charge transporting layer, in an OPC the charge is photogenerated and subsequently injected into the charge transport material dissolved in, or a functional part of, an organic polymeric material which serves as the binder. In these devices, the aim is for charge to be transported, in the absence of trapping, from the site of injection to the counter electrode, driven by the applied field.
In charge generating elements, incident light induces a charge separation across various layers of a multiple layer device. In an electrophotographic charge generating element, also referred to herein as an electrophotographic element, an electron-hole pair produced within a charge generating layer separate and move in opposite directions to develop a charge between an electrically conductive layer and an opposite surface of the element. The charge forms a pattern of electrostatic potential, also referred to as an electrostatic latent image. The electrostatic latent image can be formed by a variety of means, for example, by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, in the electrophotographic process the electrostatic latent image is developed by contacting it with an electrographic developer to form a toner image, which is then fused to a receiver. If desired, the latent image can be transferred to another surface before development, or the toner image can be transferred before fusing.
In an electrophotographic process, the photoreceptor is typically subjected to a variety of physical and chemical abuses, such as scratching, abrasive wear and exposure to chemicals, e.g., ozone and nitrogen oxides, from corona charging. Organic photoreceptors are typically easily damaged by these abuses and their useful lifetime thereby can be decreased. The surface of an organic photoreceptor can be relatively soft, so that cleaning, by blade or brush, causes scratches and abrasive wear. Unintended contacts of the surface with sharp objects may also result in scratches that necessitate photoreceptor replacement. The photoreceptor surface is also relatively permeable and its components are reactive towards the ozone and nitrogen oxides generated during corona charging. After extended exposure to such chemicals, the electrophotographic characteristics may degrade to the point where image defects become objectionable and the photoreceptor must be replaced. Organic photoreceptors are also susceptible to photochemical damage from ultraviolet radiation emitted from the corona discharge or from exposure to room light. As a result of these factors, the lifetime of an organic photoreceptor is on the order of one hundred thousand cycles. By contrast, a lifetime of one million cycles is typical of the much harder amorphous selenium and arsenic triselenide photoreceptors. Extensive efforts have been devoted in attempts to stabilize organic photoreceptors from such abuses.
Overcoating the photoreceptor with a tough and chemically impervious overcoat layer is one approach that has been utilized to extend their useful life, such as with the materials disclosed in U.S. Pat. Nos. 5,204,201; 4,912,000; 4,606,934; 4,595,602; 4,439,509; and 4,407,920. If they are used, an overcoat should desirably bind well to the underlying photoreceptor materials, not be too brittle such that it cracks in an electrophotographic process, coated in a relatively very thin layer, and transport charge to prevent unwanted charge build up during electrophotographic process cycling. The resistivity of an overcoat has important consequences in an electrophotographic system. If the overcoat has high resistivity and inadequate capability to transport holes, the time constant for voltage decay will be excessively long relative to the processing time for the electrophotographic element, and the overcoat will retain an undesirably high residual potential after photodischarge of the underlying photoreceptor. The magnitude of the residual potential depends upon the initial potential, the dielectric constants of the various layers, the thickness and the charge transport characteristics of each layer. A solution to high residual potential has been to reduce the thickness of the overcoat layer. Another solution is to provide an overcoat that is conductive. The overcoat should not, however, be too conductive. The electrophotographic element should be sufficiently electrically insulating in the dark that the element neither discharges excessively nor allows an excessive migration of charge along the surface of the element. An excessive discharge (“dark decay”) would prevent the formation and development of the latent electrostatic latent image. Excessive migration causes a loss of resolution of the electrostatic image and the subsequent developed image. This loss of resolution is referred to as “lateral image spread.” The extent of image degradation will depend on the processing time for the electrophotographic element and the thicknesses and dielectric constants of the layers. It is thus desirable to provide an overcoat that is neither too insulating nor too conductive so as to meet the objectives previously mentioned.
The triboelectric properties of the overcoat should also desirably be matched to the triboelectric characteristics of the electrophotographic toner used to develop the electrostatic latent image. If the triboelectric properties are not matched well enough, the electrophotographic element will triboelectrically charge against the electrophotographic toner. This causes disruption of the charge pattern of the electrostatic latent image and can result in background in the resulting toner image. For example, an overcoat can triboelectrically match a particular negatively charging toner, but not triboelectrically match another toner that charges positively.
Silsesquioxanes generally are a class of silicone polymers that have been used as abrasion resistant coatings, including a coating for organic photoreceptors. Such organic silicone coatings are normally prepared by a sol-gel process. Certain silsesquioxane overcoat layers for organic photoreceptors are disclosed in U.S. Pat. Nos. 5,731,117; 5,693,442; 5,874,018; and 6,066,425. The protection of organic photoconductors using an overcoat of polysiloxane mixtures in a polycarbonate resin is described in U.S. Pat. No. 6,030,736.
Charge transport materials (CTMs) have also been added to polymeric binder layers to transport charge in organic photoreceptors. These layers are in general insulators that carry charge when either holes or electrons are injected into them. U.S. Pat. No. 3,542,544 discloses triphenylmethanes and tetraphenylmethanes substituted with dialkylamines as CTMs that are incorporated into photoconductive elements. Triphenylmethane CTMs containing hydroxyaniline groups are described in U.S. Pat. No. 5,368,967. Electrophotographic photoreceptors in which triarylamine compounds with dihydroxy substituents are covalently bonded into polycarbonate resins are disclosed in U.S. Pat. No. 5,747,204. Arylamines incorporated into silsesquioxanes as acid scavengers for photoreceptors is discussed in U.S. Pat. No. 6,187,491. The incorporation of triarylamines in a functional subunit of a composition that also includes an inorganic glassy network subunit and a flexible organic subunit is discussed in U.S. Pat. No. 5,116,703. Imaging members containing hole transporting polysilylene ceramers are described in U.S. Pat. No. 4,917,980.
The incorporation of tertiary arylamines into silsesquioxane polymers for the purpose of transporting holes has been mentioned in U.S. Pat. Nos. 5,688,961; 5,712,360; 5,824,443; 5,840,816; 5,888,690, 5,905,008; 5,910,272, and 6,376,695. Another synthesis method is described in U.S. Pat. No. 6,046,348. In U.S. Pat. No. 6,517,984 by Ferrar et al., the teachings of which are incorporated herein by reference in their entirety, certain silsesquioxane compositions are disclosed containing hydroxy tertiary arylamines for hole transport. Three related U.S. Patents are U.S. Pat. Nos. 6,143,452; 6,203,692, and 6,265,122.
Recent articles in the chemical literature have discussed sol-gel networks, including silsesquioxanes, that have useful moieties, such as organic dyes, attached to the siloxane network through non-hydrolysable Si—C bonds, and equilibrium control addition through Si—O—C bonds. For example, E. Bellmann et al. reported the incorporation of a functional moiety, i.e., fluorinated tertiary arylamines and trimethoxyvinylsilane into polymer chains (Chem. Mater., 2000, Vol. 12, p. 1349); however, due to low reactivity of trimethoxyvinylsilane in radical polymerization, the amount of silane moieties incorporated into the resulting polymer is limited. Perylenes are said to be incorporated into sol-gel networks by first coupling them to the silane and then forming a sol-gel network, as described in M. Schneider and K. Mullen, Chem. Mater., 2000, Vol.12, p 352. Alternatively, a dye is said to be incorporated in the sol-gel formation process, as described in C. Sanchez and F. Ribot, New J. Chem., 1994, Vol.18, p 1007.; C. Sanchez, F. Ribot, B. Debeau, J. Mater. Chem. 1999, 9, 35.; F. Ribot and C. Sanchez, Comments on Inorganic Chemistry, 1999, Vol. 20, p 327; and T. Suratwala et al., Chem. Mater., 1998, Vol.10, p199.
However, there are several drawbacks for previously employed silsesquioxane overcoat layers. First, these silsesquioxane polymers are not entirely compatible with many commonly used organic materials employed for other functional layers in OPCs, and therefore, the overcoat layers do not bond well with other organic materials employed in such photoreceptors and easily peel off. Second, the organic silicone overcoat layers are usually brittle and crack under bending and mechanical fatigue. Third, due to the lack of charge transport properties, the silsesquioxane overcoat layers can build up high residual voltage during the electrophotographic process. Modifications of silsesquioxane materials have been developed, but generally do not overcome these weaknesses. As can be seen, it would be desirable to develop silsesquioxane polymers with improved physical and chemical properties which could be used as a sol-gel precursor for preparation of relatively hard, protective coatings having hole transport capabilities that would be desirable for use in electrographic elements.